This invention relates to a flexible thermal connection between a cryogenic refrigerator and a magnetic resonance imager (hereinafter "MRI") superconducting magnet.
As is well known, a magnet coil can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance of the magnet coil to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of 10 minutes) to introduce a current flow through the coil, the current will continue to flow through the coil due to the negligible coil resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of MRI.
In a typical MRI, the pressure vessel, which is contained within a vacuum vessel, is surrounded by a plurality of concentric heat shields. Each successive heat shield is at a slightly higher temperature than the cryogen in order to thermally isolate the pressure vessel from ambient temperatures on the outside of the vacuum vessel, which may be in the order of some 300.degree. C. higher than the cryogen temperature. The thermal shields are maintained at their cold temperatures by a cryogenic refrigerator which typically is contained within a stainless steel cold head sleeve cylinder with thermal connections between the cold head and the thermal shields. The thermal connection between the cold head and the thermal shields must be thermally conductive and efficient, and operate in the presence of vibration generated by the movement of the piston within the cryogenic refrigerator and the resultant movement of the cold head relative to the heat shields. In addition, the thermal connection must withstand the wide change in temperatures to which the assembly is subjected in cooling the superconducting magnet from ambient temperature to a temperature in the range of absolute zero (-270.degree. C.), and also must withstand, yet not affect, the strong magnetic fields present within the MRI, and the requirement of magnetic field homogeneity. Still further, the thermal connection must also withstand the thermal and mechanical forces resulting from differential expansion of materials during cool-down.
Flexible connections such as flat, braided cable have not proven to be completely satisfactory in meeting all of these overlapping and/or conflicting requirements.
Accordingly, it is highly desirable to be able to provide a flexible thermal connection between the cryogenic refrigerator and the thermal shields and/or the magnetic cartridge of a superconducting MRI magnet which operates in such a harsh environment yet which minimizes the transmission of motion and vibration to the heat shields in order to minimize the effects of the cryogenic refrigerator operating mechanism on magnetic field homogeneity and the resultant image quality of the MRI system.